Simulation Study On Structures And Characteristics Of Some Functional Materials

Computational materials and materials design combined with computer techniques are important contents in materials science. Here three main aspects of the dissertation have been achieved by computer simulations and modeling using first principle theory or molecular dynamics methods: electronic structures, conductive properties and their relationship of the cathode material LiCoO₂ and its doped compounds used in Li-ion rechargeable batteries; thermal behaviors of the systems of layered graphite intercalated with H, Li and other small molecules; the controlled formations of chemical bonds and their characteristics.1. Energy material is an important branch of materials science, and is also a research hotspot. In this dissertation, the electronic structures of LiCoO₂ and its doped compounds LiCo_0.92M_0.08O₂ (M=Ni, Zn, Mg, Al, Cr, Mn, Fe, Cu) have been studied using first principle theory based on density-functional theory (DFT) in local density approximation (LDA), based on the results of which, the electronic structures of LiCo_0.67M_0.33O₂ (M=Mg, Mn, Ni) have been also studied in the same methods. As the calculated results shown, compared with LiCoO₂, the band structures and the distributions of density of states (DOS) of the doped compounds have been changed for LiCo_0.92M_0.08O₂ (M=Ni, Zn, Mg, Cr, Mn, Fe), which indicated that the electronic conductivities of these doped compounds have been improved, while the electronic conductivities of LiCo_0.92M_0.08O₂ (M=Al, Cu) have not been improved. The same method is used for LiCoo.67Mo.33O2 (M=Mg, Mn, Ni) with more non-Co atoms doped to LiCoO₂. It is found that the electronic conductivities of the gained LiCo_0.67Mn_0.33O₂ and LiCo_0.67Ni_0.33O₂ have been improved compared with LiCoO₂, while the electronic conductivities of LiCo_0.67Mg_0.33O₂ have not. These facts are in accord with the experimental results. Thus it has been theoretically testified that after doped with Ni, Zn, Mg, Cr, Mn, and Fe in a proper amount, the electronic conductivity of the cathode material LiCoO₂ can be improved, while doped with Al and Cu the electronic conductivity of LiCoO₂ will not be improved. And it is proved that the electronic conductivities of LiCo_0.67Mn_0.33O₂ and LiCo_0.67Ni_0.33O₂ are higher than that of LiCoO₂. The improved mechanism of charge balance and compensation with Oxygen ions taken into account is firstly adopted to explain for these changes of cathode materials used in Li-ion rechargeable batteries and the calculated results may afford enlightenment and guidance for exploring and developing new type cathode materials with high properties and performances.2. Graphite is also a kind of electrode material with high quality and attracts much research attention experimentally and theoretically. The thermal behaviors for the system of graphite intercalated with small atoms or molecules like H and Li have been studied using molecular dynamics (MD) method in this dissertation. And the conductive band gaps for them have been calculated using the extended Huckel method. It is found that the rates of diffusion of both Li and H atoms increase with the increase of the simulation temperature: from 50K to 200K, and the specific diffusive behaviors and rates for Li and H are different according to their trajectories. The thermal behaviors of Li are more complex than that of H. The conductive gap is broadened by about 0.1eV for H-GIC, but it remains unchanged for Li-GIC, which indicates that the addition of Li does not influence the conduction characteristic of graphite while that of H does. These results tell the fact that Li-GIC is proposed to be a favorable material for the electrode, which is consistent with the experimental facts. At the same time, these calculated results have explained why there exists a fairly irreversible capacity of Li for graphite and other carbonaceous materials. Besides, systems of layered graphite intercalated with CO₂, H₂O and NH₃ etc. have also been studied by MD method, the results of which show that the thermal behaviors of these small molecules are similar to those of H and the lattice structures of layered graphite will be destroyed irreversibly if the quantity of these small molecules increases. This is coincident with facts.3. It is a great goal to control atoms or molecules to build up functional structures and devices. The third part of our work is following an experimental fact of the controlled formations of chemical bonds between CO and Fe/Cu adsorbed on Ag (110) using STM. The experimental fact is testified and characterized by the first principle method. And we have extended this method to other series of metal atoms adsorbed on Ag (110), such as Sc, V, Cr, Mn, Co, Ni, Zn, Zr, Ag, Au and some lanthanons. It is found that according to their electron number in the outermost layer orbital, the calculated results can be classified into three kinds, (i). With an unfully occupied d orbital, such as Fe, Co, Ni and C, structures for M(CO)/Ag(110) are corresponding to S₁ and structures for M(CO)₂ /Ag(110) are corresponding to SS₂, no stable S₂ structures are found for m(CO)/Ag(110); (ii). With a fully occupied d orbital, such as Cu, Zn, Ag and Au, the structures for both m(CO) / Ag (110) and M(CO)₂/Ag(110) are corresponding to S₃; (iii). With f orbital, such as those lanthanons, the structures for M(CO)/Ag(110) and M(CO)₂/Ag(110) are very complex because of the complicated space extending directions for f and more and deeper investigations into themechanisms for chemical bond formations between lanthanons and CO should be undertaken. The calculated results also show that the chemical bonds between these metal atoms and CO appear different characteristics, for example, different bond angles, different bond energy and different torsions because of their different space extending directions of outmost layer orbital. The frontier orbital theory (FOT) has been adopted to explain these results. All of these results help us to understand the procedures for controlled chemical bond formations and may provide enlightenments in controlling and manipulating single atoms or molecules.